Traction elevator systems typically comprise an elevator cab, a counterweight, and one or more sheaves from which the cab and counterweight are suspended. The counterweight typically weighs about the same as the elevator cab when the cab is filled to about 40% capacity.
There are certain problems in attempting to balance the weight of an elevator cab with a counterweight. For example, the counterweight typically has a static, non-varying weight, whereas the cab will always have a variable load. Therefore, the cab and counterweight will not always be in balance. This condition presents stresses on the drive motor.
U.S. Pat. No. 3,845,842 discloses a traction elevator system wherein upper chambers within the car and the counterweight receive a weight medium which is transferred between the chambers, and thereby vary the weight of the car and the counterweight in relation to changes in the loaded weight of the car. The weight medium can be a fluid or chains. However, this system has drawbacks such as the potential for fluid leakage and loud noises produced by chain movement.
U.S. Pat. No. 5,788,018 attempts to overcome balancing issues by eliminating the counterweight and relying instead on a continuous hoist rope and comparatively heavy compensation sheave mounted at the lower end of the elevator shaft. The weight of the compensation sheave and its associated support assembly is carried by the drive rope, thereby applying traction force to the rope itself sufficient to hold the car and move it to a desired position. Nevertheless, the heavy compensation sheave still functions to counterweigh the cab.
There is therefore a need for an improved elevator system wherein the weight of the cab is effectively and continuously counterbalanced to minimize the load on a drive motor.